1.4b Network Types, Area Types

Network Types 

OSPF network types control:

• How hellos and updates are sent
• Who forms an adjacency
• How the next-hop is calculated

Point-to-Point 

The simplest network type.  As the name suggests is simply a link between two routers.  A packet sent from one router, can only ever go to the one recipient on a point-to-point network. 

Broadcast 

Since Point-to-Point networks aren’t very scalable, a much more efficient way of connecting routers is on a broadcast network.  On Broadcast networks, OSPF will elect a DR and a BDR since more than two routers can participate.  

Non-Broadcast Multi-Access 

NBMA emulates the function of a broadcast network, without the broadcasting.  Every router on an NBMA network must be statically configured with neighbor relationships, and then Hello packets will be unicast to each individually configured neighbor.  DR and BDR are elected. 

Point-to-Multipoint 

An alternative approach to NBMA, but instead of trying to emulate a broadcast, it effectively collates neighbors into a group of point-to-point networks.  No DR/BDR required. 

Area Types 

Backbone	Area 0.  Must be directly adjacent to every other area (or at least have virtual link)
Normal	Connects to the backbone area through one or more ABRs.
Stub	Areas configured as a stub do not receive Type 5 LSAs (external) – instead they receive a default route from the ABR.
Totally Stubby	Taking the idea of the Stub area and going more extreme, Stub routers only receive a default route. No IA, no External routes.
Not-so-stubby	Like a stub area but will advertise external routes from a connected ASBR into the OSPF network via a type 7 LSA to the ABR which is converted to a type 5 and broadcast.
Totally not-so-stubby	Like the totally stubby area, but will advertise external routes from a connected ASBR into the OSPF network via a type 7 LSA to the ABR which is then converted to a type 5 and broadcast.